DVDs are used as a storage media for digital and digitized information. Presently, they are available in six formats: DVD-5 (5 Gbyte, prerecorded); DVD-9 (9 Gbyte, prerecorded; DVD-10 (10 Gbyte, prerecorded); DVD-18 (18 Gbyte prerecorded); DVD-R (3.8 Gbyte, recordable); and DVD-RAM (2.6 Gbyte read/write). Although, they are similar in appearance to better known compact discs or "CDs," DVDs are mastered in a different manner than ordinary CDs. Further, while CDs are made from a single substrate that is approximately 1.2 mm thick, DVDs are made from two component discs or "halves" made from optical grade polycarbonate. Each half is approximately 0.6 mm thick and 120 mm in diameter and the two halves are bonded together to form a DVD. An exemplary DVD is shown in FIG. 1. Among the advantages of DVDs over CDs is that each of the two halves may contain more than one information-carrying layer, thereby increasing storage capacity. In contrast, a CD has a single information-carrying layer.
While offering information capacity advantages over other storage media, a difficulty with DVDs is that their two halves must be bonded together. Presently, the two halves of a DVD are bonded in specialized machinery using liquid adhesives. In order to achieve an acceptable bond, the distance between the two DVD halves (i.e., the thickness of the intermediate adhesive layer) should be between 40 to 70 microns with a radial non-uniformity of about 4 microns (depending on the format specifications of the particular type of DVD being manufactured). DVDs must also be manufactured with a certain disc flatness or "tilt" (a tilt of 10 microns or more induces severe rotational disc vibrations rendering the DVD unreadable by its player). For discs in the DVD-9 format, an example of which is shown in cross section in FIG. 2, there are additional requirements.
One of the advantages of a DVD-9 disc is that its two informational layers are readable by a disc player from one side of the disc. However, in order for the disc player to read both informational layers, the laser beam from the pick-up must be able to travel through the intermediate layer of adhesive. Thus, the layer of adhesive must be optically clear and, specifically, it should be substantially transparent to radiation having a wave length from about 635 to about 650 nanometers (nm). Further, its refractive index (n) should be 1.5 to 1.6 and its single path bi-refringence should be between 30 nm to 50 nm (to be compatible with the optical grade polycarbonate of the two DVD halves).
As can be appreciated, air is often entrapped between the two halves of a DVD when it is manufactured. Yet, air bubble inclusions in the adhesive layer are not desirable and at a certain size are problematic. Air bubbles cause diversion of the disc player laser beam and even slight beam diversion can render a disc unreadable. Laser beam diversion is caused by the substantially different refractive index of air, n=1.003, versus the refractive index of polycarbonate, n=1.586. (The refraction of light at polycarbonate-air and polycarbonate-adhesive interfaces, accordingly to Snell's law, is shown in FIGS. 3B and 3C.) Air bubbles can also contribute to the delamination of a DVD's halves.
Each type of DVD has its own advantages and capabilities. Some disc formats, such as DVD-10 and DVD-9, have similar data storage capabilities. However, as can be seen by reference to FIG. 4, the informational layers in a DVD-10 disc are read from two sides of the disc. Thus, to make a DVD-10 disc comparable to a DVD-9 disc requires a dual laser beam pick-up DVD player. Such disc players are relatively expensive. On the other hand, the DVD-9 format offers the ability to have both informational layers read from one side of the disc, using a relatively inexpensive, single laser beam pick-up disc player. Thus, the DVD-9 format is one of the more attractive media for applications where an inexpensive disc player is used, but relatively large storage capacity is needed, such as, for example, interactive applications.
As noted, discs in the DVD-9 format have a relatively large capacity and can be used in inexpensive disc players. However, present DVD manufacturing methods are not satisfactory for manufacturing discs in the DVD-9 format. One method of forming a DVD disc is to use a hot-melt adhesive. As shown in FIG. 5, the hot-melt disc bonding method involves rubber-roller deposition of thermally liquefied, hot-melt adhesive to DVD halves. Adhesive is applied at 120.degree. C. to 150.degree. C. over the entire contact surface of both disc halves. In the next step, the DVD halves are pressed together to allow for uniform adhesion between the two halves (via a polymerization process). Since the disc halves are pressed together (against a flat surface), the desired disc flatness or tilt is easily achieved. However, one problem associated with DVDs formed using hot-melt adhesives is "droop."
DVDs can be exposed to high temperatures (for example, when they are on the dashboard of a car during the summer) and structural instability of the hot-melt adhesive bond can occur under such conditions. The structural instability manifests itself as a dimensional change or droop in the adhesive. Droop may cause the two halves of a DVD to become misaligned or may cause the spacing between them to become non-uniform. Droop of the hot-melt adhesive occurs at the glass transition temperature (or Tg point) of the material. The Tg point for typical hot-melt polymers is about 90.degree. C. Sometimes structural welds are used in DVDs in order to diminish drooping. However, since the entire contact surfaces of the DVD halves are coated with hot-melt polymer, structural welding of discs to prevent disc droop via ultrasonic welding can not be achieved, because the adhesive bond prevents the necessary vibration. Structural welding using other welding methods may be available.
The hot-melt adhesive bonding method suffers from an additional problem; entrapment of air. As can be seen by reference to FIG. 6, entrapment of air bubbles can occur at the three interface layers of a hot-melt, adhesive-bonded DVD. Among other problems caused by the existence of such air bubbles, the entrapped air affects the cosmetic look of a DVD. In order to hide air-bubble-cosmetic defects that occur in the adhesive layer, translucent and even colored hot-melt adhesives are used. While some attempts to remove the air bubbles rather than hide them have been made, they have not been successful. Vacuum bonding (used to eliminate air bubbles during the bonding process) can not be applied when using hot-melt adhesives because liquefied thermopolymers actively "outgas" (transition from a liquid to a gaseous state), thus affecting the bond between the two halves. The outgased chemical substances also contaminate the vacuum pumping system. Accordingly, the hot-melt adhesive method does not satisfactorily meet the requirements of DVD-9 discs. That is, using this method, it is not feasible to produce a DVD with an optically clear adhesive layer that is free from entrapped air.
The other presently used method for bonding DVDs employs UV-curable adhesives. There are two ways of achieving a UV adhesive bond: radical UV and cationic UV. The radical UV method involves simultaneous spin coating and capillary dispersion of a UV-curable polymer between the DVD halves. The cationic method involves application of UV adhesive via a screen printing process. The cationic or screen printing method has limitations similar to those in hot-melt bonding (although, since cured UV fluid is hard and dimensionally stable even at temperatures above 100.degree. C. there is no droop problem).
The radical UV method, which is illustrated in FIG. 7, requires extremely high quality, expensive DVD substrates (disc halves that have superior flatness, disc surface parallelity, and bonding surface wet-ability). Injection molding of substantially flat 0.6 mm polycarbonate substrates (including near perfect replication of data pits) is a complex and relatively "slow" process (6 to 7 seconds cycle speed). This makes the process relatively expensive and time-consuming rendering the radical UV method much less viable as a real option for most DVD formats.
Another difficulty with the radical UV method involves applying the UV-curable fluid polymer without entrapping air within it. Uniform capillary dispersion of fluid polymer with good "surface wiping" action to displace all air (bulk and surface level) while maintaining bonding layer thickness between the DVD halves is very difficult to achieve with present techniques. Efforts to improve dispersion of the fluid, including increasing the bonding surface energy of the DVD halves by an oxygen plasma pre-clean of the polycarbonate, maintaining fluid and disc temperature (to control constant viscosity of the polymer at approximately 30 to 40 centipoids), and maintaining low water content in the polymer require exacting control procedures and equipment. These complexities in combination with common UV-overcuring problems make the UV adhesive method very expensive and time consuming. However, the radical UV process is the only presently available method of manufacturing DVD-9 format discs.
Accordingly, it would be desirable to have a method and a system that overcomes these problems and by which the two halves of a DVD can be bonded together. Further, it would be desirable if the method and system were relatively simple and inexpensive.